A typical metal forming power press is disclosed in Haulsee et al. U.S. Pat. No. 4,996,865.
Metal forming power presses generally use a crankshaft, a connecting rod, an oscillating or “swing” beam, and a link to force a forming punch mounted on a ram constrained by sliding element bearings through a series of dies to form a finished product. Such a prior art metal forming press is shown in FIG. 1.
Where the shape is intricate and a large amount of metal forming is required, a progressive tooling arrangement is used to form the product. Progressive tooling formation requires that the forming punch travel through the tools in a straight line to avoid damage to the tools and to create accurate finished products. A simple slider crank arrangement creates large forces normal to the motion of the punch-carrying ram causing distortions to ram movement.
A typical prior art metal forming press uses a five bar linkage to create ram movement. An eccentric throw on a crankshaft drives a connecting rod. The connecting rod is connected approximately halfway up an oscillating beam. The upper end of the oscillating beam is attached to one end of a short link, while the other end of the link is attached to a straight line guided ram.
One method used to lessen the magnitude of the normal forces is to add additional links to the slider crank arrangement to ensure that the link attached to the sliding ram has as little angular movement as possible while driving the ram. FIG. 2 shows such an arrangement commonly used in a can forming machine positioned at back dead center.
FIG. 3 shows this arrangement at back dead center in a side view.
In this arrangement, crankshaft 7 with eccentric throw 30 rotates clockwise. Eccentric throw 30 is attached by rotating joint 31 to connecting rod 6. Connecting rod 6 is connected to the approximate center of beam 5. Beam 5 is connected via lower pivot 35 to a pivot point, stationary shaft 32, in machine frame 10. As the crankshaft 7 rotates, connecting rod 6 is driven by eccentric throw 30 and thus forces beam 5 to oscillate through a given angle. Generally, the length of oscillating beam 5 is chosen so that the arc of movement of upper connection point 33 of beam 5 passes both above and below the centerline of ram 2 by an equal distance. The upper connection point 33 is connected to link 3. Link 3 is then connected via pivot pin 34 to ram 2.
As the crankshaft 7 and connecting rod 6 drive beam 5 through its oscillating motion, link 3 drives ram 2 horizontally through front and rear sliding bearings 1. Link 3, therefore, both translates and oscillates as it moves. The oscillation is troublesome in that, due to axial forming forces, forces normal to ram 2 are created by link 3 angularity. These normal forces then cause deflection of ram 2, which in turn causes poor tool life and finished product wall thickness variation.
FIG. 4 through FIG. 11 illustrate the angularity of link 3 during the movement of ram 2.
FIG. 4 shows that at back dead center for this common arrangement link 3 must be at a 2.5 degree down angle in order to connect ram 2 with beam 5.
FIG. 5 shows the lower pivot 35 of beam 5 constrained to rotate about stationary pivot point 32 in machine frame 10.
FIG. 6 shows this same linkage arrangement with ram 2 at mid stroke on the forward stroke. Beam 5 is now positioned vertically, and upper connection point 33 is now above the horizontally sliding line of motion of ram 2.
FIG. 7 is an enlarged detail view of link 3 connecting ram 2 and beam 5. Link 3 is now at a 2.5 degree up angle in order to connect ram 2 and beam 5.
FIG. 8 shows the lower pivot 35 of beam 5 constrained to rotate about stationary pivot point 32 in the machine frame 10.
FIG. 9 shows this same linkage arrangement with ram 2 at front dead center. Beam 5 is now positioned at an angle, and upper connection point 33 is now below the horizontally sliding line of motion of ram 2.
FIG. 10 is an enlarged detail view of link 3, which connects ram 2 and beam 5. Link 3 is now at a 2.5 degree down angle in order to connect ram 2 and beam 5.
FIG. 11 shows the lower pivot 35 of beam 5 constrained to rotate about stationary pivot point 32 in the machine frame 10.
Link 3 therefore has to operate at a changing angle so as to maintain the connection between upper connection point 33 of ram 2 which is constrained by front and rear sliding bearings 1 to move in a straight line.
The pivot at the upper end of the oscillating beam traces a partial arc through space. The ram is typically positioned so that this arc falls an equal amount to either side of ram centerline.
Inertial forces from accelerating and decelerating ram 2, as well as tool forming forces, are applied to pivot pin 34 by link 3. During the machine stroke, axial forces are carried by ram 2 through pivot pin 34 and thus into link 3. Since link 3 oscillates through an angle, normal forces are created equal to the tangent of the angle multiplied by the axial force component.
These normal forces seek to bend the ram and thus create distortions to the straight line movement of the ram.
Conventional metal forming presses seek to reduce ram distortion in one of four ways:    1. Rams of large cross section with high moments of inertia are used to resist the distortion. Reynolds Metals Company Mark III presses use a 5 inch diameter tubular ram to resist the high normal forces.    2. Way systems consisting of hardened flat steel surfaces are placed all around the moving ram. The way system has either rolling element bearings or hydrostatic bearings with small clearance to the ram. The way system then supports the ram and minimizes ram distortion from the normal forces. Standun and Carnot Metal Box metal forming presses use a way system.    3. A Watts linkage can be used to generate a better approximate straight line. Early Ragsdale metal forming presses use a Watts link.    4. A Peaucellier mechanism can be used to generate an exact straight line path. Current Ragsdale metal forming presses as manufactured by Alcoa Packaging Machinery use a “Diamond” mechanism which is a class of the Peaucellier mechanism.
Each of these approaches has problems.
A large cross section ram adds appreciable mass thus causing increased parts breakage, increased axial forces, and increased normal forces.
Way systems require the ram to be manufactured with a relatively large square or rectangular feature. This feature must be manufactured flat and perpendicular so as to be capable of being guided in straight line. The feature also adds significant mass to the ram system. This square or rectangular feature is then guided by the ways. The ways are subject to high wear and tightly controlled tolerances. Wear in the way system occurs when a rolling element bearing fails or a control orifice in one of the hydrostatic bearings plugs. When a support bearing fails, the ram contacts the way system, damaging the surfaces and necessitating press overhaul and parts replacement. Highly precise flatness and parallelism are required for the way system. Therefore, highly trained personnel and highly accurate measuring equipment are required to maintain the way system.
A Watts link is an approximate straight line solution and adds duplication of the oscillating beam on the opposite side of the mechanism. Reciprocating mass is essentially doubled, increasing parts breakage and axial forces. Details of a Watts link are discussed in engineering kinematic textbooks.
The Peaucellier mechanism generates a true straight line but more than doubles the number of links, and the additional links add reciprocating mass to the mechanism. The large number of links coupled with space considerations means that the mechanism is fragile. In order to maintain a straight line path, the pin joints of the linkage must have very little clearance. Wear in the pin joints destroys accuracy and allows impact forces to destroy the links. Details of a Peaucellier mechanism are discussed in engineering kinematic textbooks.